View source for Occurrence (query directive)

== Description ==

Gets the occurrence with specified type and version from the active column value of input. You can leave out the version to get all occurrences of a type or both parameters to get all occurrences in the topic.

If you specify both the type and the version but such occurrence exists, will return null. If you leave out one or both of them, will return an empty result if no matching occurrences are found.

If you don't specify both the version parameter, then the version will be added as a column in the result row. If you don't specify the type either then that also will be added to the result row.

== Constructor ==

Occurrence([ [TopicOperand type [,TopicOperand version] ])

== Notes ==

The version is usually one of the standard language topics. You can get these using the ''getLang'' method in ''XTMPSI'' class. To use this you need to include ''org.wandora.topicmap package''. See examples on how to do this.

== Examples ==

Following example gets all instances of the input topic which have an occurrence with the value of a string literal "Test". Note the use of ''XTMPSI.getLang'' method to get the language topic subject identifier. This method can also be given ''null'' as a parameter to get the language independent topic.

 importPackage(org.wandora.query2);
 importPackage(org.wandora.topicmap);
 new Occurrence("http://www.wandora.org/occurrence",XTMPSI.getLang("en"))
 .as("#occ")
 .from(new Instances())
 .where("#occ","=","\"Test\"")



[[Category:Query directives]]